The present invention relates to magnetic field sensors and, more particularly, to a temperature compensated Hall sensor for testing magnetic recording heads for data storage systems.
Many data storage systems use magnetic recording heads for writing information to and reading information from a magnetic medium. For example, disc drives of the xe2x80x9cWinchesterxe2x80x9d type have one or more rigid discs, which are coated with a magnetizable medium for storing digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor, which causes the discs to spin and the surfaces of the discs to pass under respective head suspension assemblies. Head suspension assemblies carry transducers which write information to and read information from the disc surface. An actuator mechanism moves the head suspension assemblies from track-to-track across the surfaces of the discs under control of electronic circuitry. xe2x80x9cFloppy-typexe2x80x9d disc drives use flexible discs, which also have circular, concentric data tracks. For a tape drive, the information is stored along linear tracks on the tape surface.
In these applications, several different types of transducers have been used that rely on magnetic properties for writing to and/or reading from the magnetic medium. For an inductive-type transducer, the direction of current through the transducer is controlled during a write operation to encode magnetic flux reversals on the surface of the medium within the selected data track. When retrieving data from the medium, the inductive transducer is positioned over the data track to sense the flux reversals stored in the data track and generate a read signal based on those flux reversals. In a magnetoresistive type of transducing head, the flux reversals cause a change in the resistance of the head, which is sensed by a detector circuit. Typically, a reference current is passed through the magneto-resistive head and the change in resistance is sensed by measuring changes in the voltage across the head. Other types of detecting circuits can also be used.
In order to understand the basic physics of a magnetic transducing head during development and manufacturing, it is common to test the response of the head to an applied magnetic field. For example, one series of tests is known as xe2x80x9cTransfer Curve Testingxe2x80x9d. To generate a transfer curve for a particular transducing head, the head is placed in a magnetic field (steady state or time varying) and the output signal from the transducing head is measured. The transfer curve is simply a plot of the output signal versus the applied magnetic field, where the field is varied from some negative value to some positive value, which is usually the same magnitude as the negative value. For a magneto-resistive type of head, the output signal consists of a steady state voltage, which is a function of the bias current applied to the head, the bulk resistance of the head and the applied magnetic field.
Typical characteristics that can be measured from the transfer curve data include read signal amplitude at maximum field, noise with zero field, noise with applied field, linearity over some range of field, and symmetry. Symmetry is a comparison of the read signal amplitude with a maximum positive field and the read signal amplitude with a maximum negative field.
In order to interpret the transfer curve data accurately, it is important that the strength of the magnetic field that is applied to the transducing head be known. Hall sensors are traditionally used to measure the magnetic field in transfer curve testers. The magnetic field is typically created by passing a current through a set of coils positioned relative to the transducing head. The current passing through the coils generates heat, which increases the temperature of the Hall sensor. Since Hall sensors have a large temperature coefficient, the increase in temperature affects the output of the Hall sensor. Therefore, it is important to implement some sort of temperature compensation in a magnetic head testing devices.
Traditional methods of temperature compensation have been found to add noise to the Hall sensor output, increase the size of the Hall sensor and compromise band width. For example, in one method of temperature compensation, a temperature sensor is placed immediately adjacent to the Hall sensor to generate a temperature measurement that can be used for compensating the output of the Hall sensor. In this method, the temperature sensor should be placed as close as possible to the Hall sensor so that the temperature measurement is accurate. However, when testing a miniature transducing head, the increased size of the Hall sensor caused by the addition of the temperature sensor can prohibit the Hall sensor from being mounted directly in the area of interest adjacent to head. This can cause additional errors to be introduced if the sensed magnetic field is not identical to the field being applied to the head.
Thus, a head testing apparatus and Hall sensor having improved temperature compensation are desired.
One aspect of the present invention is directed to a head testing apparatus for testing a magnetic data recording head. The apparatus includes a test volume adapted to receive the magnetic data recording head and a magnetic field source positioned to generate a magnetic field within the test volume. A Hall sensor is positioned relative to the test volume and has bias current input terminals, voltage output terminals, an operating temperature and an impedance between the bias current input terminals, which varies with the operating temperature. A first bias current source is electrically coupled to the bias current input terminals and is adapted to provide a first bias current, which is modulated based on the impedance of the Hall sensor.
Another aspect of the present invention is directed to a method of compensating a voltage output of a Hall sensor for changes in the temperature of the Hall sensor. The method includes: applying a first bias current to bias current inputs of the Hall sensor, measuring a representation of an impedance of the Hall sensor, wherein the impedance of the Hall sensor varies with the temperature of the Hall sensor, and modulating the first bias current as a function of the measured representation of the impedance of the Hall sensor.
Yet another aspect of the present invention relates to a temperature compensated Hall sensor circuit, which includes a Hall sensor and a bias current source. The Hall sensor has bias current input terminals, voltage output terminals, an operating temperature and an impedance between the bias current input terminals, which varies with the operating temperature. The bias current source applies a bias current to the bias current inputs of the Hall sensor and modulates the bias current as a function of the impedance of the Hall sensor to reduce effects of the operating temperature of the Hall sensor at the voltage output terminals of the Hall sensor.